dec_neon: add SaturateAndStore4x4
dec_neon: add SaturateAndStore4x4

converts 2 s16 vectors to 2 u8 and store to uint8_t destination;
TransformAC3 can reuse this after a rework

Change-Id: Ia9370283ee3d9bfbc8c008fa883412100ff483d0

Aexyn/libwebp,Acidburn0zzz/webm.libwebp,yohunl/libwebp,kim42083/webm.libwebp,imazen/libwebp,ttyangf/libwebp,yohunl/libwebp,ericmckean/webm.libwebp,Maria1099/webm.libwebp,matanbs/webm.libwebp,imazen/libwebp,Aexyn/libwebp,kalli123/webm.libwebp,kalli123/webm.libwebp,jacklicn/libwebp,Maria1099/webm.libwebp,iniwf/webm.libwebp,Aexyn/libwebp,reimaginemedia/webm.libwebp,redengineer/libwebp,PKRoma/libwebp,ttyangf/libwebp,ttyangf/libwebp,jiehu5114/libwebp,jiehu5114/libwebp,iniwf/webm.libwebp,abwiz0086/webm.libwebp,zofuthan/libwebp,imazen/libwebp,appunite/libwebp,ttyangf/libwebp,Suvarna1488/webm.libwebp,iniwf/webm.libwebp,altogother/webm.libwebp,comoyo/gonzo-libwebp,appunite/libwebp,jiehu5114/libwebp,Suvarna1488/webm.libwebp,PKRoma/libwebp,kim42083/webm.libwebp,abwiz0086/webm.libwebp,Suvarna1488/webm.libwebp,rxl194/libwebp,ericmckean/webm.libwebp,Maria1099/webm.libwebp,yohunl/libwebp,redengineer/libwebp,Aexyn/libwebp,matanbs/webm.libwebp,ya7lelkom/libwebp,webmproject/libwebp,webmproject/libwebp,redengineer/libwebp,gshORTON/webm.libwebp,keane83/BLE_IM,jacklicn/libwebp,PKRoma/libwebp,abwiz0086/webm.libwebp,comoyo/gonzo-libwebp,webmproject/libwebp,zofuthan/libwebp,keane83/BLE_IM,webmproject/libwebp,webmproject/libwebp,yohunl/libwebp,jacklicn/libwebp,gshORTON/webm.libwebp,abwiz0086/webm.libwebp,matanbs/webm.libwebp,ya7lelkom/libwebp,redengineer/libwebp,jiehu5114/libwebp,zofuthan/libwebp,PKRoma/libwebp,kim42083/webm.libwebp,jiehu5114/libwebp,altogother/webm.libwebp,keane83/BLE_IM,Maria1099/webm.libwebp,altogother/webm.libwebp,rxl194/libwebp,Acidburn0zzz/webm.libwebp,abwiz0086/webm.libwebp,rxl194/libwebp,ya7lelkom/libwebp,jacklicn/libwebp,PKRoma/libwebp,kim42083/webm.libwebp,imazen/libwebp,ya7lelkom/libwebp,kleopatra999/webm.libwebp,zofuthan/libwebp,iniwf/webm.libwebp,imazen/libwebp,gshORTON/webm.libwebp,Acidburn0zzz/webm.libwebp,appunite/libwebp,reimaginemedia/webm.libwebp,ericmckean/webm.libwebp,appunite/libwebp,jacklicn/libwebp,matanbs/webm.libwebp,comoyo/gonzo-libwebp,kleopatra999/webm.libwebp,reimaginemedia/webm.libwebp,comoyo/gonzo-libwebp,ericmckean/webm.libwebp,yohunl/libwebp,kim42083/webm.libwebp,matanbs/webm.libwebp,kalli123/webm.libwebp,altogother/webm.libwebp,reimaginemedia/webm.libwebp,kleopatra999/webm.libwebp,Acidburn0zzz/webm.libwebp,rxl194/libwebp,gshORTON/webm.libwebp,ericmckean/webm.libwebp,Suvarna1488/webm.libwebp,PKRoma/libwebp,Acidburn0zzz/webm.libwebp,kleopatra999/webm.libwebp,appunite/libwebp,comoyo/gonzo-libwebp,reimaginemedia/webm.libwebp,ttyangf/libwebp,appunite/libwebp,altogother/webm.libwebp,keane83/BLE_IM,comoyo/gonzo-libwebp,Aexyn/libwebp,comoyo/gonzo-libwebp,kalli123/webm.libwebp,kalli123/webm.libwebp,Maria1099/webm.libwebp,redengineer/libwebp,iniwf/webm.libwebp,keane83/BLE_IM,rxl194/libwebp,ya7lelkom/libwebp,gshORTON/webm.libwebp,zofuthan/libwebp,Suvarna1488/webm.libwebp,kleopatra999/webm.libwebp